A typical power supply circuit receives power from an AC source. During a normal mode of operation, the power supply circuit generates either a constant voltage or a constant current to a DC load. The power supply circuit includes a first-stage power factor correction circuit that operates to maintain the current drawn from the AC source substantially in phase with the voltage of the AC source. The power factor correction circuit provides a first-stage output voltage to a second-stage DC-to-DC converter circuit. The second-stage DC-to-DC converter circuit generates the constant voltage or the constant current for the DC load.
The power factor correction circuit may be based on a boost converter or a buck-boost converter. One type of buck-boost converter is a flyback converter. A boost converter can only produce an output voltage that is greater than the input voltage. A flyback converter can also produce an output voltage that is greater than the input voltage. Unlike a boost converter, a flyback converter can also produce an output voltage that is less than the input voltage. Although both types of converters can be used in a power factor correction (PFC) circuit, a PFC converter based on a boost converter topology typically can achieve a greater power factor (e.g., closer to 1) than a PFC converter based on a flyback topology. A PFC converter based on a boost converter topology typically has a lower total harmonic distortion than a PFC converter based on a flyback technology. The foregoing is summarized in the following table for an exemplary 50-watt PFC:
TABLE IPower Factor (PF) and THD for50-watt Flyback PFC and Boost Type PFC50-Watt PFCFlyback Type PFCBoost Type PFCVINTHDPFTHDPF120 V 8%0.963%0.99277 V18%0.98%9.96
Table I illustrates that a lower power factor and greater total harmonic distortion (THD) is a drawback for using flyback type PFC converter circuits instead of boost type PFC converter circuits.
As is well known in the art, a reduced power factor results from two main contributing factors. One factor is the THD of the line current. Another factor is a phase-shift β between the line current and the line voltage. The power factor (PF) can be defined in terms of the two contributing factors as follows:
                              P          ⁢                                          ⁢          F                =                              1                                          1                +                                                      (                                          T                      ⁢                                                                                          ⁢                      H                      ⁢                                                                                          ⁢                      D                                        )                                    2                                                              ×                      cos            ⁡                          (              β              )                                                          (        1        )            
Both the flyback type PFC converter circuit and the boost type PFC converter circuit can force the input current to be in phase with the input voltage. Accordingly, the phase-shift β is not the main contributing factor in causing the flyback type PFC converter to have a poorer (e.g., lower) power factor. The experimental results shown in Table I support a conclusion that the reduction in power factor caused by input current distortion (e.g., THD) is much higher for a flyback type PFC converter in comparison to a boost type PFC converter.